Mira Nair's Vanity Fair can never quite make its mind up about Becky Sharp. During this energetic but uneven film, Becky undergoes a bewildering number of transformations. At first, Reese Witherspoon appears to be playing her as a 19th-century English counterpart to the equally ambitious and unscrupulous high school student she portrayed so memorably in Alexander Payne's Election.

As the movie progresses, we get to see Becky in all sorts of different guises. "I had thought her a mere social climber. I see now she is a mountaineer," remarks one character as Becky tries to gatecrash high society. But Nair also treats Thackeray's creation as a feminist heroine, struggling against a patriarchal society. Constantly changing outfits and with an obvious enthusiasm for English country-house living, Witherspoon's Becky bears more than a passing resemblance to Madonna. At other points, she has her Scarlett O'Hara moments. By the final reel, dressed in a black veil and making a living in gambling dens on the continent, she seems somehow to have turned into Marlene Dietrich.

For all the confusing shifts in tone and characterisation, there is plenty to admire here. The detailed production design and Declan Quinn's cinematography evoke Thackeray's London as a seething, dirty city, with pigs on the streets. Julian Fellowes (of Gosford Park fame) fills his screenplay with acerbic one-liners - some borrowed from the novel, some his own invention. And the film is full of memorable turns by familiar character actors: Gabriel Byrne's roué marquess of Steyne, Jonathan Rhys Meyers' vain popinjay George Osborne, Bob Hoskins' slovenly Sir Pitt Crawley, Jim Broadbent's snobbish miser and Geraldine McEwan and Eileen Atkins' clucking, malevolent old biddies.

At times, the storytelling verges on the baffling. The effort of compressing a thick novel into a two-hour movie has confounded the film-makers, who leave many loose ends. Nor is it clear (unless she wants to evoke her own earlier filmography) why Nair throws in quite so many references to Indian culture and society.

As Thackeray adaptations go, this Vanity Fair conspicuously lacks the intensity and consistency of tone that marked Kubrick's masterpiece, Barry Lyndon. None the less, like Becky herself, it makes up in energy and inventiveness for what it lacks in grace and subtlety.